Classically, the analysis of spectra is performed by allowing a source to emit radiation through a slit whose luminous output is first collimated, caused to fall on a usually planar diffraction grating and then focused for viewing through an image slit. Typically, in high resolution applications, the image slit position is varied in order to measure the intensity of light at various positions in the spectra, which positions correspond to various wavelengths of light in the particular emission spectra being analyzed. Typically such analysis can be done through the use of photocells which provide not only an indication of the existence of light at a particular wavelength, but also an indication of the magnitude of light at that wavelength.
A major advance in the development of high quality low aberration systems for the analysis of spectral emissions was introduced in the late 1960's by the assignee of the present application. In particular, Flammand has described in his patent the use of holographic recording techniques for the manufacture of a diffraction grating, wherein the hologram is recorded on a concave surface, presented the possibility of high quality aberration-corrected focusing gratings.
Still a more interesting possibility is discussed in later patents of the assignee herein in which a grating is disclosed in which a substantially planar spectrum is produced by a single concave spherical grating surface while minimizing to any possible degree the various aberrations such as coma, astigmatism, and so forth. As can be seen in accordance with the teachings of this patent, the calculation of the various aberrations in a grating may be made depending upon recording and use parameters.
While the essence of the above-referenced patent to Flammand was the recognition of an intuitive approach to a solution of the problem of providing an aberration corrected grating, later quantitative approaches when coupled with intuitively produced models, provided the possibility of whole families of solutions with excellent characteristics.
With the introduction of increasingly fast, and powerful computers over the last fifteen years, grating designs have been increasingly based on more of a brute force approach involving the postulation of recording configurations and the measurement of their characteristics of use and the evaluation of variations of gratings of known characteristics in order to develop gratings having similar characteristics but being somewhat tailored to particular or potential uses. Such work is done by computer modeling techniques based on models. Thus, the possibility exists today of manufacturing, through the use of holography, gratings having a wide range of characteristics, such as low coma, low astigmatism, or low spherical aberrations.
However, as a practical matter single element focusing and dispersing systems, while of very high quality, do have limitations. In particular, stray and randomly scattered radiation becomes particularly troublesome when one wishes to discriminate extremely weak emissions which are in the presence of somewhat stronger emissions For example, if one wishes to detect the presence of lead in materials containing calcium, the extreme brightness of the calcium lines will tend to create noise in the detection of the lead spectral emissions. A similar situation occurs if one wishes to detect the presence of arsenic where there are strong calcium emissions.
While the causes are not the same, virtually the same problem is encountered in the case of laser induced emissions where the sample is excited by a laser beam having a very powerful luminous output. Such techniques are used for example, in both Raman spectroscopy and fluorescence measurements. One possible solution to this problem is the use of the so-called double monochromator. The principles behind such devices are discussed for example, in an article written by M. V. R. K. Murty in Applied Optics, volume 11, no. 1, July 1972.
Generally a double monochromator comprises a first slit acting as a source including light of a particular wavelength which is the signal which one wishes to extract in addition to emissions at numerous other wavelengths which may be considered as potentially contributing to noise. This slit is arranged to cause light including the "signal" to be detected to fall upon a collimating mirror which reflects the light onto a planar diffraction grating which, in turn, diffracts the light into a second focusing reflective member. The output of the focusing reflective member is then passed through another slit which acts as a spatial filter or mask selecting out the desired wavelengths plus a certain quantum of the noise at a wavelength outside the desired range of wavelengths. This light is then passed on to a second collimating reflector which in turn causes light to fall upon a second planar diffraction grating. The light is then reflected onto a second focusing mirror which passes light to an output slit. Thus, in principle, the output of the double monochromator has a much higher signal to background ratio than the input to the double monochromator. Nevertheless, the double monochrometer comprises quite a few rather inexpensive components.